Optical particle detection systems are useful for applications such as contamination control, which is critically important in the manufacturing processes of several industries. These industries may require cleanrooms or clean zones with active air filtration, as well as a supply of clean raw materials such as process gases, deionized water, chemicals, and substrates. For example, in the pharmaceutical industry, the Food and Drug Administration requires particulate monitoring because of the correlation between detected particles in an aseptic environment and viable particles that contaminate the product produced. As another example, semiconductor fabrication companies require particulate monitoring as an active part of quality control.
Optical particle detection systems utilize illumination to determine the presence, size, number and/or concentration of particles in a volume. The particles are typically suspended in a fluid, which may flow through the volume that is illuminated and viewed by the detection system. The basic components of such a system are a laser illumination source; a view volume in which the particles to be detected may reside and into which the illumination is directed; and one or more sensors, which are typically photodetectors, that can detect optical disturbances of the illumination caused by the particles in the view volume.
The illumination source in an optical particle detection system is typically a semiconductor diode laser. The source ideally is stable, quiet, and has lone life. There are two basic types of diode lasers used in particle detectors: gain-guided and index-guided. Gain-guided lasers are typically quiet enough for a particle detection system because at any given time they have many longitudinal modes running, which allows for an averaging effect over all the modes. When one mode turns off and another turns on, as for instance when the temperature or drive current changes, the average light is relatively constant. Suitable gain-guided lasers, however, are short-lived, having a typical useful life of approximately sixteen to eighteen months of continuous operation in the absence of effective countermeasures.
Index-guided lasers, on the other hand, typically have a much longer useful life, with a mean-time-to-failure as long as eight years. Index-guided lasers, however, suffer from a problem associated with mode hopping. Unlike a gain-guided laser, an index-guided laser typically operates in one dominant longitudinal mode of lasing. When used in continuous-wave mode, index-guided lasers often switch modes or mode-hop as the temperature or other external conditions change, and they are therefore not sufficiently stable for use in a particle detection system. The mode hops can create noise levels higher than the relative intensity noise (RIN) in the laser beam, thereby causing false counts. Furthermore, as a laser ages or when the operating temperature increases, more drive current is required to maintain constant power. While that fluctuation can be compensated for with the use of an automatic power control circuit, raising the drive current increases the junction temperature, which consequently increases the laser beam's wavelength. Unfortunately, the wavelength shift is not gradual; the wavelength will abruptly change from one mode to another at specific operating conditions, and frequency instabilities and excess noise can occur at those mode-hopping points. To combat those ill effects, one class of known prior art uses active temperature compensation and a constant laser drive current to stabilize the laser beam's wavelength in areas between mode-hopping points.
Mode hopping therefore directly imposes a serious limitation on the usefulness of an index-guided laser and, by necessitating the use of shorter-lived gain-guided lasers, indirectly imposes a serious limitation on the useful life of a laser in an optical particle detection system. The consequent frequent replacement of lasers in such systems represents a significant cost and burden to the users of such systems.
To make matters worse, laser mode hopping is especially problematic when the sensor is of the light-extinction type. Sensors in optical particle detection systems come in two varieties: light-scattering and light-extinction. Light-scattering sensors, as the name implies, detect scattered light from a particle when it passes through the laser beam. A standard particle detection system operating according to the light-scattering principle passes a fluid sample stream containing the particles through an elongated flattened nozzle such that the sample stream exiting the nozzle intersects a laser beam in a view volume. Scattered light from particles in the view volume is collected with optics and processed to determine such things as particle counts or sizing information. On the other hand, light-extinction sensors detect the amount of loss in a laser beam when the particle passes through it. Unlike light-scattering type sensors, which can be designed to minimize the amount of detectable background light and therefore minimize the amount of detectable mode-hopping noise, light-extinction type sensors are typically illuminated directly by the laser beam, so any small change in the laser light, such as mode-hopping noise, is very likely to be detected as false counts. That is because a typical sensor is sensitive to light level changes on the order of nanowatts, such as when a particle passes through the laser beam. Thus, any instabilities or RIN in the laser beam may be detected as false particle counts, as an increase in the RIN from the laser can cause the sensor's output signal to exceed the shot noise generated by background light impingent on the sensor.
In contexts other than particle detection systems, it has been known to decrease mode-hopping noise by modulating an index-guided laser, so as to cause multi-mode lasing. That technique has been applied successfully in videodisc players, for example. Although Japanese Patent Application No. 09-1786645 describes the same technique used in a light-scattering type optical particle detection system, to our knowledge, no one has successfully utilized that technique in an optical particle detection system of the light-extinction type.